The present invention relates to a surface-shape-defect inspecting method and its apparatus for optically detecting shape defects of an internal surface of a plane member with its circumference composed of a member having good rigidity and shape defects of a surface of a plane member having a thin and plane plate shape such as a mask used as a screen for printing a circuit pattern and semiconductor paste in a process of manufacturing devices such as an electronic-circuit board and hybrid ICs by identifying positions of the shape defects.
FIG. 7 is a skeleton diagram showing a squint view of an apparatus for printing a circuit pattern or semiconductor paste on a substrate by using a screen 1. As shown in the figure, the screen 1 is provided with an external frame 4 built along the circumference thereof as a single assembly in order to make the screen 1 rigid and the work to handle the screen 1 easy to do. It should be noted that the screen 1 is a thin plane plate member made of a nickel plate 2 with a thickness of several tens of microns and pattern holes 3 are bored through it in an etching process. When necessary, a knitted-in mesh described below is attached to the screen 1 itself. In the case of a screen for creating a pattern with a large numerical aperture or the like, by attaching a knitted-in mesh made of stainless wires each with a diameter of several tens of microns, the strength of the screen 1 as a whole can be enhanced depending upon the shape of the pattern.
When screen printing is carried out by the screen 1 having a configuration described above, while ink 6 in a paste or liquid state is being rubbed on the screen 1 by using brush hair called squeegee 5, the ink 6 is pushed out from the pattern holes 3 so that a pattern 9 determined by the shape of the pattern holes 3 is transferred to substrate 8 mounted on the holder 7.
By the way, in a screen printing process, the screen itself is required to have highly accurate flatness for a reason to be described later. Thus, it is necessary to check the flatness of the screen itself prior to the screen printing. These requirements also apply to a plane member for a special purpose other than the screen even if no pattern holes are bored through the plane member.
It should be noted that, so far, surface shapes and defects of a plane member for example are inspected and detected in order to achieve some objectives. A method for detecting the unevenness displacements of the surface of a plane member is disclosed in documents such as Japanese Patent Laid-open No. Sho62-127614. According to this method, a laser beam radiated by a semiconductor laser is applied to a work surface by way of a radiation-side light converging lens while a light reflected by the work surface is detected by a detector through a light converging lens on the light-receiving side. A work-surface displacement is detected from the light receiving position on the detector. According to a method disclosed in Japanese Patent Laid-open No. Hei1-250705, on the other hand, predetermined processing is carried out to measure the shape of the three-dimensional surface, an object of measurement. An example of the predetermined processing is scanning by a line-shaped slit light along straight lines over the entire area of the three-dimensional surface.
Even if a large waviness deformation on a screen as a whole can be corrected during an operation to transfer a circuit pattern to a substrate by using a squeegee, screen printing is carried out with local deformations and uneven portions existing on the screen by keeping their deformed states as they are. As a result, in actuality, the improvement of the printing accuracy can not be expected.
This is because gaps are formed inevitably between the screen and the surface of the substrate subjected to printing, losing a state of adherence during the printing operation. As a result, it is feared that the accuracy and reliability of the printing on a printed matter is lost, causing printing blurs to be generated or the printed matter to be injured. In order to solve this problem, the surface of the screen which is used as a printing negative is required to have such excellent flatness that no local deformation or unevenness is formed on the surface, in particular, the surface of the side in direct contact with the printed matter. For this reason, it is thus necessary to take countermeasures wherein the flatness of the surface is evaluated in advance and local deformations and uneven portions found in the evaluation are corrected or an uncorrectable bad screen is discarded.
It should be noted that, in the case of a member with holes and/or openings for creating a pattern are formed on the surface thereof such as a mask used as a printing screen, with the methods disclosed in the documents described above, not only is it possible to measure waviness and unevenness on the surface of the member, but a large waviness deformation and a local deformation on the surface of the member can not even be identified.